A switching amplifier, also known as a class-D amplifier, has an output that is switched or pulse-width modulated at a frequency much higher than the frequency of interest. A class-D amplifier is generally significantly more efficient than a class-AB amplifier and is especially suitable for applications in portable electronics devices, e.g., portable compact disc players, digital video disc players, MP3 players, etc. However, a traditional class-D amplifier requires an output filter, which would significantly increase the size, weight, and cost of the amplifier. In addition, a class-D amplifier may use a bridge output stage topology that switches in a binary fashion. In such a switching process, there is always a current in the load. When there is no input signal, the amplifier operates near zero crossing and the majority of the current used is wasted, resulting in a drop in efficiency.
A signal modulation scheme may enable a class-D amplifier to operate without an output filter. The scheme includes delivering current to the load only when needed, and once delivered, maintaining the current, instead of removing the current that results in the energy loss. When an input signal is present, the resulting differential signal across the load consists of narrow pulses with polarity determined by the polarity of the input signal. These narrow pulses also double the differential PWM frequency. This then achieves the desired effect of placing current into the load only when needed, resulting in less energy loss in the load and therefore and increased efficiency.
Eliminating the output filter in a class-D amplifier causes the amplifier to radiate electromagnetic interference (EMI) from the wires connecting the amplifier to the load. A change in current generates a magnetic field, while a change in a voltage results in an electric field, both of which may cause EMI. The electric field, which is a common-mode effect, could be significant if the load, e.g., speaker, wires are long. EMI negatively affects the operation of the amplifier and other electronic devices. Furthermore, EMI radiation decreases the efficiency of the amplifier.
Accordingly, it would be advantageous to have a signal modulation scheme that substantially eliminates EMI in the operation of a filterless class-D amplifier. It is desirable if the signal modulation scheme does not introduce signal distortion while eliminating EMI. It is also desirable to have a class-D amplifier that is energy efficient. It is further desirable for the class-D amplifier to be simple and reliable. It is would be of further advantage for the amplifier to be cost efficient.